The present invention relates to a rolling bearing used under high temperature conditions and, more in particular, it relates to a rolling bearing suitable to engine auxiliaries such as alternators, solenoid clutches, intermediate pulleys, compressors for vehicle air conditioners and water pumps.
Along with downsizing and weight reduction of automobiles in recent years, higher performance and higher power as well as down-sizing and weight reduction have been demanded also for the engine auxiliaries. For example, during operation of an engine, high temperature, large vibrations and heavy load (about 4G to 20G as gravitational acceleration) caused by high speed rotation exert simultaneously by way of a belt to bearings for use in an alternator. As a result, flaking occurs in an early stage to the rolling bearing, particularly, to an outer ring as a fixed ring and this tends to shorten the bearing life.
The prior art intended for the improvement of the life of bearings used under large vibrations and heavy load includes, for example, Japanese Examined Patent Publication No. Hei 7-72556 (hereinafter also referred to as prior art 1), Japanese Patent No. 2724019 (hereinafter also referred to as prior art 2), Japanese Unexamined Patent Publication No. Sho 62-218542 (hereinafter also referred to as prior art 3) and Japanese Unexamined Patent Publication No. Hei 2-190615 (hereinafter also referred to as prior art 4). Among them, the prior art 1 discloses that plastic deformation due to the decomposition of residual austenite under the raceway surface can be prevented by defining the amount of residual austenite in the outer ring of the bearing to 0.05% or more and 6% or less at least on the side of load input, that is, on the side of the pulley. Further, the prior art 2 discloses a heat resistant bearing steel of a composition comprising 0.8 to 1.5% C, 0.8 to 2.0% Si, 0.3 to 2.0% Mn, 1.3 to 1.98% Cr and 0.3 to 1.0% Mo, the total of Si and Mo being within a range of satisfying 1.0% or more, and the residue of Fe and impurities. Further, the prior art 3 discloses that the bearing ring contains 0.95 to 1.10% C, 1 to 2% Si, 1.15% or less of Mn, 0.90 to 1.50% Cr, and 8% or less of the amount of residual austenite, and has a surface hardness HRC of 60 or more. Further, the prior art 4 discloses a grease-sealed bearing having grease sealed in the bearing, in which an oxide layer of 0.1 to 2.5 xcexcm thickness is formed on the rolling surface of the bearing ring of the bearing.
By the way, as a countermeasure for preventing early flaking in a bearing used under high temperature, large vibrations and heavy load caused by high speed rotation, [SAE Technical Paper: SAE 950944 (held in Feb. 27-Mar. 2, 1995)] describes in first to 14th sections that early flaking can be prevented by analyzing the fatigue mechanism of the bearing for use in the alternator, and replacing the sealing grease from E grease to M grease having a higher damper effect thereby absorbing large vibrations, heavy load and moderating metal contacting.
For the early flaking phenomenon, it is considered that water contained in a lubricant (sometimes contained, for example, usually by about 0.1% in grease) is decomposed and created hydrogen ions are adsorbed to the raceway surface, and accumulated as hydrogen atoms to a high strain area (near the maximum shearing stress position), which lead to stress corrosion cracking flaking. Further, for the cause of the formation of the water contained in the bearing, it is considered that since the auxiliary is heated to a high temperature during operation of an engine and cooled to an atmospheric temperature after stopping the engine, air presenting in a slight space of the bearing is condensed.
On the contrary, it is disclosed in the prior art 1 that tempering (at 250 to 380xc2x0 C.) is conducted such that the mount of residual austenite at least in the outer ring on the side of the pulley is from 0.05 to 6%. However, mere decreasing in the amount of residual austenite can provide an effect for the improvement of the dimensional stability under a high temperature circumstance but this can only suppress the plastic deformation under the raceway surface for the flaking resistance and since sliding of the rolling element increases under large vibration, heavy load, the effect of prolonging the life is not recognized in a circumstance where hydrogen intrudes from the raceway surface.
Further, the prior art 2 also discloses that a hardness capable of withstanding the rolling contact fatigue can be maintained even during use at high temperature by adding an element of increasing the resistance to temper softening such as Si or Mo but macro carbide precipitates such as Cxe2x80x94Cr tend to be formed to remarkably worsen the crack extension characteristic since C is contained up to 1.5% and Cr is contained by 1.3 to 1.98%. Further, no life extend effect can be expected by merely adding Mo by a small amount as 0.1 to 1.0%, since this can not precipitate fine Mo type carbides for improving the lower limit amplitude value of stress intensity factor.
Further, in the prior art 3, since the amount of residual austenite is decreased to 8% or less by high temperature tempering with addition of an element of improving the resistance to temper softening of steel such as Si or Al, a bearing ring with less dimensional aging change under a high temperature circumstance can be provided. However, for the flaking resistance, since sliding of a rolling element increases under large vibration, heavy load, the effect of prolonging the life can not be recognized under a circumstance where hydrogen intrudes from the raceway surface.
Further, in the prior art 4 since a troublesome treatment of dipping a bearing ring in an aqueous solution of sodium hydroxide under heating at a low temperature is necessary for forming a tri-iron tetroxide layer (generally referred to as a black coating) and, further, a treatment of corroding the rolling surface to such an extent as causing coloration in an aqueous oxidizing solution such as alcohol sulfate, hydrochloric acid or sulfuric acid as other solvent it involves problems, for example, in view of the facility or the processing time. Further, as shown in xe2x80x9cPretext for Conference of Japan Triopology (Tokyo, 1995-5) p 551-554xe2x80x9d, in a bearing for use in engine auxiliaries used under large vibrations/heavy load, since auto-rotation slip is caused at the inlet of a fixed ring, oxide layers capable of providing a damper effect are ruptured and a load exerts directly on the outer ring frequently suffering from early flaking, so that it is actually difficult to prevent early flaking in the fixed ring.
Further, when a bearing is heated in air without controlling the temperature, scales of several xcexcm (skins) are formed on the surface of the material by oxidation. Unevenness of the scales causes loss of metal to possibly form initiation points for pits. Further, when the bearing is merely left in atmospheric air, it may be considered that steels react with moisture in air to possibly cause atmospheric corrosion.
Further, the rolling contact fatigue is a phenomenon caused by shearing stress and vertical compressive stress synthesized under the rolling surface, and cracks are extended also in a synthesized mode of a tensile crack extension mode (mode I) and a shearing crack extension mode (mode II). Accordingly, for determining crack extension characteristic data for bearing materials, a study on bearing materials excellent in crack extension resistance characteristic has also been conducted, for example, by conducting a crack extension test using a compression/tensile (CT) test piece by a test method according to ASTM E 647-83.
The present invention has been developed in order to solve the foregoing problems and it is an object thereof to provide a rolling bearing used under a high temperature circumstance at 150xc2x0 C. or higher, preferably, 180xc2x0 C. or higher as in engine auxiliaries, by dispersingly precipitating Mo type or Ti type carbides of 50 to 300 nm finely on the surface of a bearing ring to obtain a hydrogen trapping effect, thus enabling to prevent hydrogen from intruding to the vicinity of a maximum shearing stress position where a rolling contact fatigue is greatest, thereby increasing the effect of prolonging the rolling life under high temperature, large vibrations and heavy load, or enabling to improve a lower limit amplitude value of the stress intensity factor by a pinning effect of Mo type carbides of 50 to 300 nm size described above, thereby suppressing extension of fine initial cracks.
For attaining the object, a rolling bearing in accordance with the present invention has a feature in which a plurality of rolling elements are disposed and used between bearing rings comprising a fixed ring and a rotational ring of a rolling bearing, wherein at least one of the bearing rings contain: C=0.65-1.25%, Si=0.7-2.5%, and Cr=0.5-3.0%, and contains at least one of: Mo=0.5-3.0% and Ti=0.05-0.5% in which Mo type carbides or Ti type carbides with an average grain size of 50 to 300 nm are dispersingly precipitated finely. Then, hydrogen deleterious in the lowering of the life is fixed only at the rolling surface and can be prevented from intruding as far as the maximum shearing stress position to obtain an effect of prolonging the rolling life by the hydrogen trapping effect of the Mo type or Ti type carbides in the constitution. Further, the lower limit amplitude value of stress intensity factor is improved by the pinning effect of the fine Mo type carbides in the constitution to obtain an effect of suppressing the extension of initial fine cracks thereby retarding progress to flaking and prolonging the rolling contact fatigue life .
Among them, C is an element for providing a hardness required as a rolling bearing, in which a hardness HRC 58 or more required for the rolling bearing can not sometimes be ensured if it is less than 0.65% and, on the other hand, if it is contained in excess of 1.25%, precipitated carbides tend to become huge to sometimes deteriorate the contact fatigue life and resistance to impact load, so that it is defined as: C=0.65-1.25%.
Further, Si is an element improving the retardation for the change of tissue, hardenability and resistance to temper softening but the effect is not sufficient if it is less than 0.7%, whereas the machinability is remarkably deteriorated if it exceeds 2.5%, so that it is defined as: Si=0.7-2.5%.
Further, Cr is an element improving the hardenability and promoting the sphericalization of carbides and it is necessary to be incorporated by at least 0.5% or more, preferably, 1% or more. However, Cxe2x80x94Cr type carbides sometimes become coarser to increase the average crystal grain size and deteriorate the machinability if it is incorporated in excess of 3.0% and, particularly, the average grain size of the carbides tends to increase to lower the amplitude value of stress intensity factor if it exceeds 1.15%, so that it is defined as Cr=0.5-1.15%.
Mo is an element improving the bearing hardness and also improving high temperature strength due to the resistance to temper softening and the dispersing effect of fine carbides and it is necessary by 0.5% or more. However, since the effect is saturated and the machinability is possibly deteriorated rather if it exceeds 3.0%, it is defined as Mo=0.5-3.0%. Further, the number of dispersed precipitates of the Mo type carbides is preferably 10 or more per 10 xcexcm2. Further, the grain size of the Mo type carbides at 50 to 300 nm is controlled by adding Mo by 1.1% or more and applying a solid solution treatment, and they can be increased to 20 or more per 10 xcexcm2. Further, since the dispersing precipitation effect has a function of improving the amplitude value of the stress intensity factor, it is preferably defined as: Mo=1.1 to 3.0%.
Ti is an element finely dispersing in the form of Ti carbides and Ti carbonitrides to improve the hardness and the rolling life of a bearing and suppressing growing of austenitic crystal grains during quenching and it has also an effect as a hydrogen trap. However, if it is 0.05% or less, Ti nitride of 1 xcexcm or more is predominant and no refining effect can be expected. Further, if it exceeds 0.50%, machinability is lowered or the number of inclusions (TiN, TiS) that lowers the rolling life increases, so that the content is defined as: Ti =0.05-0.50%. Further, for fine dispersive precipitation of TiC and TiCN, it is preferred that a solution treatment is applied at 1150 to 1350xc2x0 C. to control the size of precipitates.
Further, dispersive precipitates of Mo type or Ti type carbides of 50 to 300 nm are preferably by the number of 10 or more per 10 xcexcm2.
Further, referring to 0, it is desirably at 10 ppm or less in order to decrease the formation of oxide type inclusions. Also for S, it is preferably 0.02% or less with the same reason. For the amount of residual austenite, it is defined as 1% or less by tempering at a temperature of 400xc2x0 C. or higher, considering the dimensional change due to decomposition of austenite, in a working circumstance at a high temperature of 150xc2x0 C. or higher. Since the presence of austenite even by a small amount causes dimensional change, the amount of residual austenite is preferably 0% that causes no dimensional change. Those with no dimensional change after tempering at a temperature of 300xc2x0 C. or higher were adopted as 0% content.
The surface of the bearing ring or the rolling element may be applied with an ordinary heat treatment, that is, a tempering treatment at 400 to 550xc2x0 C. after quenching and a heat treatment may be applied again to 200-400xc2x0 C. in air after grinding, to provide the surface hardness HRC 58 or more and the amount of residual austenite to 1% or less, and form an oxide layer comprising an iron oxide type materials of 5 to 100 nm (also referred to as re-heated oxide layer), thereby attaining a hydrogen intrusion inhibitive effect by the re-heated oxide layer, so as to prevent fracture of the layer formed with a lubricant on the raceway surface due to the effect of heavy load and large vibration, suppress intrusion of hydrogen caused by decomposition of the water content in the lubricant, thereby preventing stress corrosion cracking flaking and, further, prolonging the rolling life.